Spencer Wells is risking life and limb to collect DNA from the most isolated, remote peoples on the planet. Five years, 100,000 samples, and 40 million dollars later, he'll have a new road map to human history. By Michael Shnayerson

WALKING THROUGH TIME: Spencer Wells, accompanied by a party of Bushmen, treks across a dry watering hole in northern Namibia.

Spencer Wells knows exactly where he wants to go next: the Tibesti mountains. He wants to fly to Libya, now that it's open to Westerners again, then hail a camel caravan across the Libyan desert to Chad, where the seven inactive volcanoes of the Tibesti rise 11,000 feet (3,353 meters) from the central Sahara: a private world of crags and chasms seldom seen by more than a handful of outsiders. Like any intrepid traveler, he's unfazed by the prospect of deadly North African windstorms and burning desert heat. The land mines near the border of Libya and Chad do pose a problem, but local guides can thread a path. As for the fierce and isolated Tubu, who've ruled the Tibesti long enough for Herodotus to have named them the Troglodytes, they're the real payoff. Wells wants to learn their oral history, how their ancestors exacted tribute from traders passing to and from the Middle East, and what this crossroads reveals about one of Earth's earliest cultures on the continent where all human life began. Then he wants to stick a cheek swab into each of their mouths to collect a generous gob of DNA-rich saliva.

Wells, 36, is a population geneticist using science in global pursuit of the greatest story not yet told: the story of how humankind traveled from its origins in Africa to populate the planet. The most telling clues lie with isolated, indigenous tribes like the Tubu, for their DNA remains, in a sense, the purest. Their unique genetic markers, characteristic mutations in a defined sequence of DNA, are like flags waving from the place their ancestors have inhabited for thousands of yearsthe starting point for ancient migrations. Any venturesome Tubu who crossed the Sahara to see the outlying world, and propagated in the process, passed on one or another of those genetic markers to his or her offspring. Any traveler who came through the Tibesti and intermarried did the same. Wells might take a cheek swab from an investment banker in Boston and find that same genetic marker: proof that one of those Tubu created a family line that leads, in some circuitous way, over continents and generations, from the Tibesti to an oak-paneled office in Back Bay. It's in the hope of tracing myriad journeys such as this that Wells, a newly named National Geographic Explorer-in-Residence, is undertaking one of the most ambitious and expensive research adventures in the National Geographic Society's 117-year history: the grandly named Genographic Project.

At a cost of 40 million dollars over five years, the brunt of it borne by National Geographic, IBM, and the Waitt Family Foundation, the Genographic Project under Wells's direction is establishing 11 DNA-sampling centers around the world, with the goal of collecting 100,000 cheek swabs or blood samples from mostly indigenous peoples like the Tubu. A sense of urgency infuses the project: Year by year, at an ever quickening rate, the outside world is crowding in on, and at the same time absorbing, indigenous peoples. A Tubu who moves to Paris will still have the genetic markers that distinguish him as a Tubu, but the geographical context for his markers will be gone. As for the Tubu who remain in the Tibesti mountains, they may marry more with outsiders as modern technology makes contact more likely. Generation by generation, tracing the last routes of historical migration of such isolated people grows that much harder. Wells wants to map as many routes as he can while their geographical origins are relatively intact.

Wells has a nifty and novel idea to help fund and publicize the project. "We're taking this directly to the people," he declares. "Because in addition to doing this work with indigenous populations, we're going to be offering for sale to the public, in the developed world mostly, the opportunity to do this cheek-swab test to see how they fit into the family tree." For about a hundred dollars, a contributor gets his or her very own cheek-swab kit along with a map of migratory routes, as Wells has charted them thus far, that is like an explorer's parchment map of the New World. "Because these participation kits are totally anonymous, there's no way anyone can find out anything about your history except you," Wells says. "Once the results are ready, you can access the Web site (www.nationalgeographic.com/genographic) for extensive details about genetics, archaeology, history, and the context for genetic variation. Your sample, if you choose, can be put into our database, so that it adds to this increasing data set about genetic variation all over the world. But when you purchase your cheek-swab kit, you're also funding research, and part of the money will be channeled back to taking samples from the 100,000 people."

For a man just weeks from the public announcement of this global gambit, Wells is forgivably a bit tense on an early spring day in his fourth-floor office at National Geographic's Washington, D.C., headquarters. Ruddy and fita whole lot more fit than your typical laboratory-bound scientisthe radiates a steely cool, like a field marshal on the eve of battle. The inevitable layman's queries about genetics elicit crisp details about mitochondrial DNA and Y chromosomes in gleaming, perfectly formed paragraphs. Most of us talk in analog; Wells is digital. Only a framed picture on the shelf suggests that not all goes according to plan. Wells's two young daughters are with their mother in Geneva; a divorce is under way. For ten years of fieldwork around the world, Wells is paying a human toll.

To Wells, the Genographic Project is a perfect double helix of history and science, the origins of which trace, for him, to a university science lab in Lubbock, Texas, which became a second home when he was all of nine years old. That was when Wells's mother, a professor at Texas Tech University's medical school, took time off to earn a Ph.D. in biology and let her son hang out while she performed her experiments. The same year, 1979, Wells was also strongly influenced by English polymath James Burke's ten-part television series Connections, with its anecdotal braiding of science and history. After earning a B.S. in biology at 19, Phi Beta Kappa, from the University of Texas at Austin, Wells took his own Ph.D. at Harvard University in evolutionary geneticsthe historical side of the science, as he saysand studied fruit flies with world-renowned population geneticist Richard Lewontin.

Fruit flies had served as an ideal test case since the early 20th century, when Thomas Hunt Morgan used them in his Fly Room at Columbia University to show how chromosomes worked, to prove that chromosomes were made up of genes, and to show how genes were passed down. But at the end of the day, Wells says, "I'm not that interested in fruit fly history. But I am interested in human population history. I wanted to apply some of those methods to human history."

"What I appreciate about Spencer is that he is not the kind of scientist who is only interested in his favorite molecule or DNA mutation," says fellow geneticist and Genographic Project coordinator Lluis Quintana-Murci of the Pasteur Institute in Paris. "Many scientists tend to be closed off in their little rooms. He always had much broader interestshuman biology and history. And he uses genetics as a tool to unravel the past."

For that, Wells went to Stanford University to work with the "grand old man" of human population genetics, Luca Cavalli-Sforza, who is now a chairman of the Genographic Advisory Board. He wanted to learn what the bold new field of genome sequencingidentifying every gene in a living thing and mapping its relation to every other genecould do for tracing human history.

For years, scientists had studied blood for genealogical clues. Blood characteristics suggested the different cultures of a family tree. But blood was little help for telling when a Middle Eastern people, say, might have migrated to western Europe and intermarried. What the scientists needed, Wells says, was a clock.

That clock came in the form of DNA. The seemingly endless ribbons of DNA found in human hair, saliva, bloodany cell in the bodyare made up of three billion individual units, known as A, C, G, and T. Subtle variations in that sequence are what genetically distinguish one person from another. As incredibly able as the human system is at replicating each unique sequence from one generation to the next, a small number of variations, or mutations, do occasionally crop up. By analyzing specific regions of the DNA, comparing the results to known reference sequences, and identifying differences that are anthropologically significant, geneticists are able to track mutations.

"They're like spelling mistakes," Wells says. "Imagine you're copying a very long document, and occasionally you'll put an A where there should be a C. And that mistake has been translated down through the generations, and more mistakes have accumulated. So the longer the lineage has been in existence, the more mistakes the sequence is going to have. And if you know the rate at which those mistakes occur, you can actually estimate how long this individual has been evolving since that origin, how long his DNA has been accumulating changes."

Initially Cavalli-Sforza's team focused on something called mitochondrial DNAa good choice because it appears frequently in the cell, so it's easy to find and track. "It's effectively the remnant of a bacteria that became engulfed by the cell about a billion years ago," Wells explains. "And all higher organisms have these structures in their cells." As Cavalli-Sforza's group knew, mitochondrial DNA is inherited strictly maternally. A mother passes it to her son, but the son can't pass it on. Only from mother to daughter does it keep descending, generation to generation. In most African tribes, women did the traveling, mostly to find mates, while the men stayed put. So the story that mitochondrial DNA told was only half of the story. Cavalli-Sforza's geneticists needed a piece of male DNA they could study to prove their theory of African origins and migration. That was when the team's researchers began studying variations in the Y chromosome, which is passed down from fathers to sons and had, up until then, been a maddeningly inscrutable bit of DNA.

By studying how mutations had accumulated in both mitochondrial DNA and Y chromosomes and determining the rate at which those mutations occurlike counting tree ringsthe geneticists made a dramatic conclusion: The populations with the greatest number of mutations were in sub-Saharan Africa. They had the oldest living lineages, which meant they were, beyond the shadow of a genealogical doubt, directly related to the earliest of our traceable ancestors. Their DNA marked the spot where humankind began.

Archaeology had suggested this, of course. But what the geneticists saw from their DNA sequencing of current-day Africans is that their ancestors appeared to have lived as the only humans on Earth as recently as 60,000 years ago. That was when they started migrating, taking their genetic mutations with them, and passing them down. Why did they leave when they did? Because 10,000 years before that, one of the Pleistocene epoch's worst cold snaps nearly drove humankind to extinction, and these were the survivors, whose better brains made them more adaptable.

If Steven Spielberg were making the movie, a fur-garbed posse of these plucky migrantsthe first adventure travelers!would set off together at dawn across the savanna, covering miles each day in search of fresh prey. In fact, the "Great Leap Forward," as anthropologist Jared Diamond puts it (in a facetious borrow from Mao Zedong), probably occurred less than 10,000 years ago. The intellectual leap, Diamond and Wells contend, came first: A few children born with higher intelligence and better communication skills, capable of fashioning better tools, passed their superior genes to offspring who came to dominate their clans.

Smarter humans learned to hunt certain species. As those species migrated a few miles every year or so, or died out on the clan's home turf, the hunters pursued them. Drought accelerated these trends by causing animals to disperse more widely. Eventually, a few migrants reached the Indian Ocean and adopted a fishing life. Little by little, they migrated north up Africa's east coast. Some traversed the Red Seaperhaps on simple log rafts. Others headed farther north to the Mediterranean. By 45,000 years ago, as garbage dumps from the time attest, hunters with relatively sophisticated tools were ensconced on the Mediterranean's shores, engaged in art and other shows of complex culture, and curious about the unknown lands that lay before them in every direction.

Wells wanted DNA samples to fill in this tantalizingly sketchy picture of humankind's origins. Where had the first of those migrating Africans gone? What routes had they taken, in just tens of thousands of years, to populate the rest of the world?

For the complete profile on population geneticist Spencer Wells and the Genographic Project, pick up the August 2005 issue of Adventure.